Is MixRGB ALWAYS RGB calculated?
My question is, does mixRGB always handle color inputs as RGB values and not as HSV?


Yes. HSV is somewhat of a crippled colour model.


  I know that is the way it works for most of the blending types, but are there any loopholes or exceptions that I need to be aware of?


Beware because most of them are display referred legacy blend modes conjured up in the dark Adobe days. The math is available in the Adobe PDF specification. They are busted up garbage in a scene referred, physical light transport system.